"rustup doc" fails silently when docs are not installed

[rspeer]

I installed Rust on macOS 10.10 using rustup, and tried running rustup doc to get to the local documentation, as some recently-written instructions told me to. Nothing happened.

I Googled and found that I needed to first run rustup component add rust-docs. After that, rustup doc did what was expected, opening the local documentation in Chrome.

It would be nice if rustup doc could say in a message that the docs need to be installed with rustup component add rust-docs, instead of just saying nothing.

[rspeer]

Sorry, I searched for this issue and didn't find it, but now I see it's a duplicate of #901.